1. Field of the Invention
Embodiments of the present invention relate to systems and methods for capacitive measurement using a rheometer. More particularly, embodiments of the present invention relate to systems and methods for measuring the rheological properties of a sample on a rotational rheometer, while exhibiting a reduced sensitivity to motions that are not in the direction or axis of the desired measurement.
2. Background of the Invention
Rotary rheometers, viscometers or viscosimeters are typically used to measure rheological properties of materials, such as their viscosity, compliance, and modulus, by rotating, deflecting or oscillating a measuring geometry in a material, either by applying a torque and measuring the resultant velocity or displacement, or by applying a velocity or displacement and measuring the resultant torque. The torque and velocity/displacement are used in conjunction with measuring geometry factors to determine the properties of the material.
As used herein, the term “rheometer” includes rheometers, viscometers, viscosimeters and similar instruments that are used to measure the properties of fluid or similar materials (see list below). Viscosity is an internal property of a fluid that offers resistance to flow (i.e., it concerns the thickness of a liquid).
The term “material,” as used herein, includes liquids, oils, dispersions, suspensions, emulsions, adhesives, biological fluids, polymers, gels, pastes, slurries, melts, resins, powders or mixtures thereof. Such materials are also referred to herein as “fluids.” More specific examples of materials include asphalt, chocolate, blood, drilling mud, lubricants, oils, greases, photoresists, liquid cements, elastomers, thermoplastics, thermosets and coatings.
A common use for a rheometer is to determine fluid properties of a material. One technique is to apply a torque developed by a motor in the presence of the material, and measure the resultant velocity or displacement. The torque and velocity/displacement are used in conjunction with measuring geometry factors to determine the properties of the material. Thus, the rheometer requires a position sensor that is extremely accurate, linear, stable and consistent. The position sensor must operate over a very small range of motion with a high resolution of position.
Unfortunately, current rotary rheometers use sensors that suffer from gain error. Specifically, mechanical motions that are not in the axis of the primary measurement cause parasitic capacitance that can be reported erroneously as a change of the primary measurement axis. Thus, it is desirable to create small-angle or small-displacement capacitive sensors that have greatly reduced sensitivity to typical sources of mechanical positioning error.